A persistent network issue shut down the MAX system Tuesday morning, the agency said.

The system was brought to a standstill just before 11 a.m. by an issue that affected computer-aided communications between operators and the TriMet's central dispatch center.

Trains were rolling again by 11:25, but at slower speeds. The agency warned that riders should expect "major delays."

High temperatures later in the day are also expected to cause delays. MAX trains travel slower when temperatures top 90 degrees, and the agency said its MAX trains were unlikely to get back on schedule before those restrictions kick in.

"Our biggest concern right now is the riders out there," TriMet spokeswoman Roberta Altstadt said. "They are experiencing major delays throughout the system, and we definitely want to apologize to them."

Arrival-time displays at MAX stations might not be showing correct times, Altstadt said. TriMet's Transit Tracker system was showing arrival times only, as well as by text and phone.

Similar computer issues had affected MAX, as well as the transit agency's website and payment systems, over the weekend.

The issues began when a planned network equipment upgrade went awry, shutting down all the transit agency's computer systems. A newly installed component was faulty, Altstadt said.

That delayed the start of service Saturday and Sunday mornigns. It also prevented ticket vending machines on MAX platforms from processing credit and debit card transactions and blocked customers from adding money to their Hop electronic fare cards on Saturday.

An attempt to make a permanent fix Tuesday brought down the communications systems, Altstadt said. The issue was expected to be resolved later Tuesday, but that would likely overlap with speed restrictions during high heat.

The repairs weren't expected to cause more delays, Altstadt said.

"We're definitely going to have a full-scale review and put processes in place so this doesn't happen again," she said.